Pierce
Pierce is a statistic representing the maximum number of Bloons that can be affected by a single projectile or attack. If a projectile loses all its pierce, the projectile is destroyed, and triggers an "on-expire" effect (if applicable). An area-of-effect attack always prioritizes the closest bloons, which consume pierce first.
Certain attacks have "impacts" (e.g. bullets from Sniper Monkey, including Bouncing Bullet), which cannot be boosted by external pierce buffs. In reality, these attacks simply have a "maximum pierce".